NMR exchange dynamics studies of metal-capped cyclodextrins reveal multiple populations of host-guest complexes in solution

Metal-capped molecular hosts are unique in supramolecular chemistry, benefitting from the inner cavity's hydrophobic nature and the metal center's electrochemical properties. It is shown here that the paramagnetic properties of the metals in lanthanide-capped cyclodextrins (Ln-alpha-CDs and Ln-beta-CDs) are a convenient NMR indicator for different populations of host-guest complexes in a given solution. The paramagnetic guest exchange saturation transfer (paraGEST) method was used to study the exchange dynamics in systems composed of Ln-alpha-CDs or Ln-beta-CDs with fluorinated guests, revealing multiple co-existing populations of host-guest complexes exclusively in solutions containing Ln-beta-CDs. The enhanced spectral resolution of paraGEST, achieved by a strong pseudo contact shift induction, revealed that different molecular guests can adopt multiple orientations within Ln-beta-CDs' cavities and, in contrast, only a single orientation inside Ln-alpha-CDs. Thus, paraGEST, which can significantly improve NMR detectability and spectral resolution of host-guest systems that experience fast exchange dynamics, is a convenient tool for studying supramolecular systems of metal-capped molecular hosts. Utilizing lanthanide-modified cyclodextrins and implementing the 19F-paraGEST method, up to three different co-existing populations of Ln-beta-CD-guest complexes were experimentally identified, despite having similar thermodynamic and kinetic properties.

Automated summary

Metal-capped molecular hosts with paramagnetic properties have been shown to be convenient for studying host-guest complexes in supramolecular systems using NMR spectroscopy. The paraGEST method, which utilizes paramagnetic guest exchange saturation transfer, allows for enhanced spectral resolution and the identification of different populations of host-guest complexes. This method is particularly useful for systems containing lanthanide-capped cyclodextrins, as it reveals multiple co-existing populations of complexes with different orientations of the guest molecules.